The present invention relates to a method of producing a spark plug.
A spark plug used to ignition of an internal engine of such as an automobile generally comprises an insulator made of alumina based ceramics disposed inside of a metal shell to which an earth electrode is fixed, and a center electrode disposed within the insulator. The insulator is axially projected out of a rear opening side of the metal shell, and has a terminal metal fixture disposed in the interior of the projected portion, which terminal metal fixture is connected to the center electrode through a resistor or a conductive glass seal layer which is formed in a glass sealing procedure. Thus, by supplying a high voltage through the terminal metal fixture, a spark is produced over a gap between the earth electrode and the center electrode.
However, under some combined conditions that, for example, temperature of the spark plug is high and environmental moisture becomes high, even a high voltage supplied fails to cause the spark over the gap, but instead, a so-called flashover occurs, that is, a discharge occurs between the terminal metal fixture and the metal shell as running around the projected portion of the insulator. Therefore, almost all of generally used spark plugs have a glaze layer formed on the surface thereof for preventing the flashover phenomenon. On the other hand, the glaze layer also serves to smoothen the surface of the insulator, prevent contamination or increase chemical and mechanical strength.
In case of glazing over spark plugs which are attached to engines, the glaze temperature becomes higher than that of a normally insulating porcelain. Further, in recent years, being accompanied with high performance of engines, ignition voltage supplied to the spark plug has been high, and the glaze has been demanded to have an insulating property endurable against severe environments.
On the other hand, in general as to automobile engines, a rubber cap is utilized to attach the spark plug to the electric system of engines. In order to heighten the flashover resistance, adhesion between the insulator and the interior of the rubber cap is important. In this case, it is required that the glaze layer to be formed on the surface of the insulator is generally free of unevenness and is smooth to the utmost.
For forming the glaze layer on the surface of the insulator, the following procedures are generally adopted. At first, glaze materials are mixed to be a predetermined composition, and then the composition is melted by use of a crucible, and is rapidly cooled for solidification to be a vitreous glaze composition. The glaze composition is finely pulverized by means of a ball mill, and is finally reduced into a glaze slurry of glaze particles suspended in a water solvent. The glaze slurry is coated to the surface of the insulator by way of spraying, spin coating or dipping, and is dried, followed by a glaze baking treatment.
Herein, in case of obtaining a glaze layer of little unevenness in thickness, it is significant that the glaze slurry has fluidity. The slurry of good fluidity spreads well by coating over the surface of the insulator and easily brings about the glaze layer of smooth and uniform thickness. On the other hand, a slurry of bad fluidity will be biased on the surface of the insulator and will result in a coat of uneven thickness.
The glaze for spark plugs contains SiO2 (silicon oxide) as an indispensable component for forming a vitreous skeleton, and for properly lowering the glaze baking temperature so as to enhance the fluidity while melting, the glaze generally comprises a silicate glass mixed with various sub-components such as boric acid (B2O3), alkaline metal components (Li2O, Na2O, or K2O), or alkaline earth metal components (MgO, CaO, SrO, or BaO). Among the components, SiO2 itself has scarcely problem because of low solubility to a water. However, some of the mixed sub-components may be of comparatively high solubility. (hereinafter referred to as called xe2x80x9cwater soluble glaze componentxe2x80x9d) The representative ones are alkaline metal oxides (Li2O, Na2O, K2O), or boric acid (B2O3).
In case of using the glaze of a high containing rate of these components, when wet-pulverizing the glaze with the water solvent, or storing it under a state of the water solvent slurry, the water soluble solvent glaze components gradually elute into the solvent. This elution gives changes to a rheology of the slurry suspending the glaze particles, so that viscosity of the slurry increases as a time passes, and the fluidity of the slurry is spoiled to easily make the above mentioned problem the uneven thickness in the coating. For example, even at an elution level which is almost a matter of no problem under a condition of the glaze layer after baking tho glaze, but if the slurry is reduced into a fine powder (average diameter is, for example, around 5 to 45 xcexcm) which can be suspended as the slurry, the influence of elution is especially difficult to be neglected.
In case of the alumina insulator for spark plugs, a lead silicate vitreous glaze was conventionally used where the silicate glass was mixed with a comparatively much amount of PbO to lower the softening point. However, with the global concern of environment protection having grown up in the recent years, the glaze containing Pb has progressively come to be out of consideration. For example, in the automobile industry using many spark plugs, considering adverse influences to environment by waste spark plugs, an investigation has been progressed to abolish use of spark plugs using Pb containing glaze in a future. However, when planning leadless spark plugs, it is unavoidable to use an increased amount of alkaline metal component or boric acid component for the purpose of restraining the rise of melting point which is caused by elimination of the lead component. As a result, the viscosity of slurry inevitably increases as the time-passing as a problem particular in consideration of the leadless glaze.
The problem of the invention is to effectively avoid the uneven glaze coat on the surface of the insulator owing to tho viscosity rising in the glaze slurry, and in turn it is an object of the invention to provide a method of easily producing a spark plug excellent in flashover resistance.
The invention has been provided to solve the problems as mentioned above, and relates to
a method of producing a spark plug, wherein an insulator of alumina based ceramics is disposed between a center electrode and a metal shell, and a glaze layer as a main of oxide is formed to cover at least part of the insulator, characterized by the steps of
preparing glaze particles to be coated, respective surfaces of which are covered with coating layers for suppressing (restraining) elution of water soluble components contained in the glaze,
preparing a glaze slurry by suspending the glaze particles to be coated in a water solvent,
piling the glaze particles for forming a glaze particle piled layer by coating the glaze slurry to the surface of the insulator, and
baking the glaze particle piled layer to the surface of the insulator by heating the insulator to form the glaze layer.
According to the invention, prior to suspending the glaze particle in the water solvent to make a slurry, the surface of glaze particle was covered with a coating layer for suppressing elution of the water soluble components contained in the glaze, thereby to obtain a glaze particle to be coated which is used to prepare a slurry, so that the water soluble components in the glaze (for example, B, F, Li, K, Na) do not easily elute into the water solvent in the slurry, and the inconvenience that the slurry viscosity rises as the time-passing may be effectively restrained. As a result, when coating the glaze slurry to the insulator, a proper coat thickness may be easily obtained, and dispersion in the coated thickness is little. Thus, the proper thickness of the glaze layer formed by the glaze-baking and reduction of dispersion in the thickness may be effectively attained. Namely, it is possible to easily produce the spark plug having the smooth and uniform glaze coat, and in turn excellent in the flashover resistance.
The effect of forming the elution-suppressing coating layer is remarkably exhibited when the glaze contains a total amount of 30 to 60 mol % of one or two or more sorts selected from a group of water soluble components of B, P, Li, K and Na in terms of B2O5, P2O5, Li2O, K2O, Na2 respectively. The adoption of such glaze composition is required particularly in case of obtaining the glaze containing 1 mol % or less of Pb in terms of PbO (hereinafter referred to as xe2x80x9cleadless glazexe2x80x9d) by way of the glaze-baking at a temperature almost the same as the range of conventional glaze baking temperature (900 to 1100xc2x0 C.) Because if the Pb containing amount is less than 1 mol % and in case the amount of the water soluble component is less than 30 mol %, the glaze baking temperature undesirably becomes too high, and if exceeding 60 mol %, the water resistance of the glaze layer itself after the glaze-baking (in case of the alkaline metal component, voltage resistance in addition to the alkaline metal component) is not be sufficiently secured.
Next, the insulator of generally available spark plugs has a projection radially extending from the outer periphery at the middle portion in the axial direction thereof, and it has also a forward portion extending toward a forward end of a center electrode in the axial direction, and further has a base portion of a body portion adjacent the projection at the rear side thereof, the base portion having an outer periphery formed as a cylinder. The glaze layer is formed in a manner of covering the outer periphery of the base portion, For enhancing the flashover resistance, the maximum height Ry is preferably 10 xcexcm or less and smooth in view of the surface roughness curve measured at the outer periphery of the base portion in accordance to the method as prescribed by JIS:B0601. The spark plug producing method of the invention is effective for obtaining such a smooth graze layer. In case the maximum height Ry is more than 10 xcexcm in view of the surface roughness curve measured at the outer periphery of the base portion, the leadless glaze layer of the above mentioned glaze composition fails to form the smooth and uniform glaze baking surface, and the close contact between the glaze baked surface and-the rubber cap is spoiled to make the. flashover resistance. The maximum height Ry is preferred if being smallest possible in view of the close contact and the flashover resistance. However, if excessively reducing the maximum height Ry, it leads to a cost-up. It is, therefore, required to seek the maximum height Ry within a range of not causing any problem (for example, Ryxe2x89xa70, 0.5 xcexcm). More preferably, the maximum height Ry is 1 to 4 xcexcm. Incidentally, the values of Ry is obtained in a manner that a reference length is picked out, in the direction of an average line, from a roughness curve measured with an evaluating length decided by JIS:B0601 (1994), and the space between the peak line and the bottom line of the picked out portion is measured in a vertical magnification direction of the roughness curve, and the values are indicated in micrometers (xcexcm) Selection of the evaluated length and the reference length is as prescribed by 4, 1, 3 of JIS:B0601.
Further, it is preferable that the film thickness of the glaze layer covering the outer periphery of the base portion of the body portion is 10 to 50 xcexcm. According to the study by the inventors, it was found out that as to borosilicate glass based- or alkaline borosilicate glass based leadless glaze layer, adjustment of the film thickness of the glaze layer is significant in obtaining the smooth surface of the glaze layer. Further, it was found out that since the outer periphery of the base portion of the body portion of the insulator is required to closely contact the rubber cap, the adjustment of film thickness, if properly conducted, will increase the flashover resistance. In case the film thickness of the glaze layer the ranges 10 to 50 xcexcm, the close contact may be obtained between the glaze baked surface and the rubber cap without lowering the insulating property of the glaze layer, and additionally the flashover resistance may be obtained.
In case the thickness of the glaze layer in the insulator is less than 10 xcexcm, it is difficult to form a uniform and smooth glaze baked surface in the leadless glaze layer of the above mentioned composition (however, excepting that the outer periphery of the base part of the insulator is smoothed by means of barrel polishing). On the other hand, in case the thickness of glaze layer exceeds 50 xcexcm, the insulating property in the leadless glaze layer of the mentioned composition is lowered, simultaneously resulting in lowering of the flashover resistance. Further, in case the insulator is glazed as it stands, the glaze sags when coating the glaze slurry or baking the glaze, and the film will become thicker as it goes down, causing difficulty to attach the cap. More preferably, the thickness of the glaze layer is 10 to 30 xcexcm.